THE MICROSCOPK 19 



which was liable to be increased by the number of 

 glasses, in the same way as chromatic aberration. 

 This defect also is increased in Compound Micro- 

 scopes ; and formerly, the two things operated so 

 greatly to the prejudice of this instrument that it 

 was seldom or never used. 



Gradually, however, means of improvement were 

 discovered. These defects were rectified in tele- 

 scopes; and at last a solution of all the diflaculties 

 that beset the path of the Microscope-maker was 

 afforded by the discoveries of Mr. Joseph Jackson 

 Lister, a gentleman engaged in business in London, 

 who, in a paper published in the PhilosojMcal 

 Transactions for 1829, pointed out the way in 

 which the Compound Microscope could be con- 

 structed free from chromatic and spherical aberra- 

 tion. This is done by such an arrangement of the 

 lenses in the object-glass, that one lens corrects the 

 defects of the other. Thus, in object-glasses of the 

 highest power, as many as eight distinct lenses are 

 combined. We have, first, a triplet, composed of 

 two plano-convex lenses of crown-glass, with a 

 plano-concave of flint-glass between them. Above 

 this is placed a doublet, consisting of a double 

 convex lens of crown, and a double concave one of 

 flint-glass. At the back of this is a triplet, which 

 consists of two double convex lenses of crown- 

 glass, and a double concave one of flint placed be- 

 tween them. Such are the combinations necessary 

 to correct the defects of lenses when employed in 

 Compound Microscopes. 



It is this instrument, then, which is most com- 

 monly employed at the present day, and to which 

 we are indebted for most of the recent progress in 

 microscopic observation. 



In using the Microscope, a great variety of acces- 

 sory apparatus may be employed to facilitate the 



